Apertured ball valves are well known for controlling the flow of a fluid through a bore, particularly in the oil and chemical process industries.
In an apertured ball valve, the valve operation may be broken down into two separate stages; firstly, the ball moves between an open and a closed position by rotating through 90° such that the ball aperture moves from an orientation coaxial with the flow direction, i.e. when the valve is open, to a position whereby the ball aperture is perpendicular to the flow direction. Secondly, the valve seals in the closed position to prevent flow through the bore across the ball valve.
A common type of conventional ball valve is the trunnion mounted ball valve in which the ball element is positionally constrained inside the valve, usually by radial bearings. The ball is rotated by the application of torque to the trunnion. Sealing occurs as a result of the valve seat floating onto the ball element. A disadvantage of this type of ball valve is that seal reliability is reduced because the sealing force only develops in proportion to the annular area of the valve seat. Thus, when trunnion mounted ball valves are used in high pressure wells and especially in “aggressive” wells in which the well fluid has a high proportion of particulate matter, the pressure is such that the particulate matter may cause degradation of the sealing surfaces of the ball and the valve seat, resulting in the valve not achieving adequate sealing integrity.
Another type of conventional ball valve is known as the floating ball valve. In this type of valve the ball is not positionally constrained relative to the valve body. Rotation is caused by the application of force to a point that is offset from the ball centre, which, in conjunction with the mating curvatures of the ball and seat, causes the ball to rotate. Sealing occurs as a result of the ball floating onto the valve seat. The disadvantage of the floating ball valve is the rotational reliability is reduced as the friction effects between the ball and seat are considerably larger than that of trunnion mounted devices. With aggressive wells, the reliability of the floating ball valve creates a problem in that the valve often seizes between the open and the closed position giving rise to serious problems in both operational and safety terms.